1. Field of the Invention
The invention relates to a method of operating a marine liquefied natural gas (LNG) carrier in which LNG is carried in at least one tank, wherein the method includes removing gas generated by evaporation of the LNG within the at least one tank, feeding the gas to at least one gas consuming prime mover of the LNG carrier, and providing power, with the at least one gas consuming prime mover, to at least one thruster. The at least one thruster is capable of consuming full power at all times notwithstanding actual thrust requirements of the LNG carrier.
The invention also relates to a method of operating a marine liquefied natural gas (LNG) carrier in which LNG is carried in at least one tank, wherein the method includes removing gas generated by evaporation of the LNG within the at least one tank, feeding the gas to at least one gas consuming prime mover of the LNG carrier, increasing a pressure of a first portion of the gas, re-liquefying the first portion, and feeding the first portion of the gas to spray nozzles arranged within the at least one tank.
The invention also relates to a method of operating a marine liquefied natural gas (LNG) carrier in which LNG is carried in at least one tank, wherein the method includes removing gas generated by evaporation of the LNG within the at least one tank, feeding the gas to at least one gas consuming prime mover of the LNG carrier, providing power, with the at least one gas consuming prime mover, to at least one electric generator mounted to the LNG carrier, and supplying electrical current from the at least one generator to at least one electric propulsion motor of a tugboat. The tugboat provides propulsion to the LNG carrier.
The invention also relates to a method of operating a marine liquefied natural gas (LNG) carrier in which LNG is carried in at least one tank, wherein the method includes removing gas generated by evaporation of the LNG within the at least one tank, feeding the gas to at least one gas consuming prime mover of the LNG carrier, providing power, with the at least one gas consuming prime mover, to at least one electric generator arranged on the LNG carrier, and supplying electrical current from the at least one generator to a tugboat via at least one flexible cable. The electrical current is sufficient to meet electric power demands of the tugboat at least when the tugboat is providing propulsion power to move the LNG carrier through water.
The invention also relates to a marine liquefied natural gas (LNG) carrier including at least one tank storing the LNG, at least one LNG gas consuming prime mover providing power to at least one thruster, an arrangement for removing gas generated by evaporation of the LNG within the at least one tank, an arrangement for feeding the gas to the at least one gas consuming prime mover, the least one thruster consuming full power of the at least one gas consuming prime mover at all times notwithstanding actual thrust requirements of the LNG carrier.
The invention also relates to a marine liquefied natural gas (LNG) carrier including at least one tank storing the LNG, at least one LNG gas consuming prime mover, an arrangement for removing gas generated by evaporation of the LNG within the at least one tank, an arrangement for feeding the gas to the at least one gas consuming prime mover, an arrangement for increasing a pressure of a first portion of the gas, an arrangement for liquefying a first portion of the gas, and an arrangement for feeding the first portion of the gas to spray nozzles arranged within the at least one tank.
The invention also relates to a marine liquefied natural gas (LNG) carrier including at least one tank storing the LNG, at least one LNG gas consuming prime mover, at least one electric generator mounted to the LNG carrier, an arrangement for removing gas generated by evaporation of the LNG within the at least one tank, an arrangement for feeding the gas to the at least one gas consuming prime mover. The at least one gas consuming prime mover provides power to the at least one electric generator. The at least one generator is structured and arranged to supply electrical current to at least one electric propulsion motor of a tugboat when the tugboat provides propulsion to the LNG carrier.
2. Discussion of Background Information
Natural gas, when cooled to approximately −260° F. changes phase from a gas to a liquid. In this state, it is called Liquefied Natural Gas or LNG. During this cooling process, the volume required to hold a specific quantity of natural gas is reduced approximately 600 times making it possible to transport significant, and economic, quantities of natural gas over great distances from source to market.
LNG is increasingly being utilized to effect the transportation of natural gas from its source in remote regions of the world to end users in population centers where demand for energy, particularly natural gas, is continually increasing. LNG can be transported in highly specialized ships or LNG Carriers (LNGC(s)). These vessels are very large and expensive and rely on transporting large volumes of LNG to achieve economical transportation rates.
Because LNG is transported in large quantities to achieve economical transportation rates over long distances, LNG receiving terminals are also quite large and situated in strategic locations for delivery of natural gas by pipeline to high demand areas. At the receiving terminals, the LNG is offloaded as a cryogenic liquid at −260° F. by pumping from the LNGC to the land based tanks of the receiving terminal. From the terminal storage tanks the LNG is then pumped by high pressure pumps to vaporizers where heat is added to return the natural gas to a gaseous state at pipeline pressures. Thereafter, it is sent out to users through traditional natural gas pipeline systems.
This approach serves the demand of gas consumers with access to the distribution pipelines, in most situations. It does not serve the demands of those gas consumers that are remote from the pipelines or those consumers connected to pipelines that have limited transmission capacity such that during high demand peak periods the pipelines cannot physically handle the quantity of natural gas being demanded.
Alternatively, some of these remote/peak customers can be more fully served by redistributing smaller quantities of LNG from a larger receiving terminal utilizing a smaller LNG vessel in the form of a lower cost non-self-propelled LNG carrier or LNG barge. Such an LNG barge would be loaded with LNG at a large LNG receiving terminal by pumping LNG from the terminals land based tanks to the LNG barge. Once loaded the LNG barge would be pushed to its destination site where its cargo of LNG would be offloaded, vaporized and processed as necessary. The resulting natural gas would then be distributed by local pipeline(s) to the customers. This approach is not dependent on long distance transmission/distribution pipelines and can, therefore, serve gas customers accessible by suitable water routes in remote areas or where peak demands cannot be met by existing distribution pipelines.
An LNG barge and such a distribution system is described in an Article entitled World's first Commercial LNG barge by Donald W. Oakley from OCEAN INDUSTRY NOVEMBER 1973, pages 29-32 is hereby expressly incorporated by reference in its entirety. This is the only LNG barge built to date and was operational for only a short period of time in 1974 when it delivered a total of only 6 LNG cargoes.
LNG is a boiling cryogen at atmospheric temperatures and pressures. It is stored and transported in heavily insulated tanks. Although heat inflow to the LNG is significantly reduced by the tank insulation, it cannot be entirely eliminated. Consequently a quantity of cold natural gas vapor (boil off) is constantly being generated and must be removed from the tank and disposed of in order to prevent over pressure of the LNG tank.
Unlike other gaseous fuels such as propane and butane which can be stored as a liquid at atmospheric temperatures by allowing the liquid and the gas in the tank to reach a stable equilibrium vapor pressure for any given atmospheric temperature, LNG (principle component methane), due to its low critical point pressure (673+psia for methane), critical point temperature (−115.8° F. for methane) and very high vapor pressures, cannot be maintained as a liquid under pressure at atmospheric temperatures.
The resulting boil off is either vented to the atmosphere (limited, by regulation, as an emergency/extraordinary procedure only, as natural gas is flammable and considered a green house gas); heated, pressurized and sent to the gas distribution system (in the case of land based LNG tanks); re-liquefied and returned to the tank as LNG; flared as waste gas; burned in the propulsion machinery (in the case of LNGCs) as fuel or contained in the LNG tank for a finite period of time by allowing the cargo tank vapor space to increase in pressure as the LNG continues to boil. This later option can only be sustained for a relatively short period of time, typically days (generally less than a month).
In the case of the only LNG barge to be built (reference 1) the boil off was allowed to accumulate in the tank by allowing the pressure in the tank to increase over time. The tanks and insulation system was designed to contain the boil off for a period of 45 days before the LNG tank relief valves would open due to overpressure releasing the natural gas to the atmosphere.
A significant problem with this approach is that the LNG itself will significantly rise in temperature to reach the equilibrium temperature corresponding to the pressure of the tank. As the tank pressure rises the LNG temperature will also rise. If this warm LNG is then subsequently pumped into an LNG storage tank that is at lower/normal pressure (slightly above atmospheric pressure ˜+100 millibars) the warm LNG will rapidly vaporize and release large volumes of cold natural gas as the LNG is cooled by evaporative processes until it again reaches equilibrium to the new tank pressure. This is unacceptable as the receiving terminal will be unable to dispose of the excess gas and tank overpressure is likely with subsequent release of natural gas to the atmosphere. Even a slightly warmer LNG can be problematic due to the phenomenon of “roll-over” resulting in rapid and uncontrolled LNG vaporization.
Large, self propelled LNGCs use the boil off as propulsion fuel in the ship's engines and are, therefore, able to maintain proper LNG tank pressure and LNG temperature. Since a barge does not have propulsion engines, this is not an option in the prior art.
The LNG barge described in the above-noted Article solved this problem of increasing LNG temperature with time by cooling the LNG in a controlled fashion during the discharge operation prior to the LNG being pumped into the land based tanks. This process is described in this Article and will not be repeated here.
This cooling process, depending on the length of time the LNG was aboard the barge and other factors, can result in discharge delays and considerable additional expense. It also significantly complicates the discharging operation. Finally, this added LNG cooling equipment is costly to purchase and expensive to maintain.
A further problem with handling natural cargo boil off by allowing the tank to pressurize is the resulting increase in the equilibrium pressure/temperature relationship as the LNG warms from the heat inflow. This creates an increased risk of extremely rapid phase transition from liquid to gas if the cargo is suddenly released from a breach of the cargo tank. This significantly increases the risk associated with the cargo of LNG.
FIG. 5 of U.S. Pat. No. 2,795,937, the disclosure of which is hereby expressly incorporated by reference in its entirety, discloses a transfer of boil off gas from the cargo tanks of a barge to a tugboat towing the barge using a flexible hose or other transference device. The boil off gas is then used as fuel in the tugboat's engines in much the same manner as in a self propelled LNGC (ship). There are many problems associated with this concept, however, which have prevented it from being reduced to practice. The most significant problem is the high likelihood of the hose being severed by the relative motion and forces between the barge and tug, and the resulting release of highly flammable natural gas. Additionally, barges are typically towed hundreds of feet behind the tugboat requiring an extremely long hose that would be subject to excessive forces, damage and failure.
U.S. Pat. No. 3,864,918, the disclosure of which is hereby expressly incorporated by reference in its entirety, discloses a method of partial liquefaction of LNG boil off gas by compression and heat exchange with cold LNG boil off gas. The method taught differs from the instant invention in a number of ways in that, among other differences, a much simplified heat exchanger/condenser is employed, there is no need for a liquid level control system and expansion of the re-condensed LNG takes place within the LNG cargo tank maximizing the beneficial effect from the Joule-Thompson cooling as a result of the expansion of the re-condensed LNG to further reduce the LNG cargo tank pressure. According to this prior art document, the required expansion of the re-condensed LNG takes place external from the LNG cargo tanks and, therefore, the resulting Joule-Thompson effect cooling resulting from the expansion of the re-condensed LNG is less effective in directly reducing LNG cargo tank pressure. Further, the flow that is returned to the LNG cargo tank(s) will, as a result of the external expansion, be a two phase mixture of vapor and liquid versus a single phase liquid flow, returning vapor to the cargo tank along with LNG versus LNG alone.
The invention(s) disclosed herein addresses the problems associated with LNG boil off gas in an LNG barge, of the type discussed above, in a number of different and novel ways. The methods disclosed herein may be applied singularly or in combination as the circumstances dictate.